Industrial sponge roller device having reduced residuals

ABSTRACT

An industrial sponge device and a method of manufacturing the same. The sponge is made from a polyvinyl acetal material with a process in which the pores are formed by gas to provide an open pore structure having no fibrils. Additionally, the sponge is purified by an extracting process which includes alternating exposure of the partially cross-linked sponge to solutions of high pH and low pH. The sponge is sterilized by electron beam radiation to destroy micro organisms that can cause contamination of the final product. The resulting sponge material is formed into an industrial sponge having a cylindrical body and a plurality of projections extending from the body. The projections are tapered truncated cones having a cross-sectional radius that decreases with distance from the body. The extracting process permits residual amounts of calcium, zinc, and other elements to be 2 ppm or less, preferably 1 ppm or less.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional of prior U.S. patent application Ser.No. 09/535,162 filed Mar. 24, 2000, now U.S. Pat. No. 6,793,612 which isa continuation of U.S. patent application Ser. No. 08/850,772 filed May2, 1997, and now U.S. Pat. No. 6,080,092, which is acontinuation-in-part of U.S. patent application Ser. No. 08/780,895filed on Jan. 9, 1997, and now U.S. Pat. No. 6,027,573, which is acontinuation-in-part of U.S. patent application Ser. No. 08/319,005filed on Oct. 6, 1994, and now abandoned, the disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an industrial sponge used for cleaningsurfaces. More specifically, the invention is a synthetic industrialsponge made of polyvinyl acetal, and a method for extracting residuefrom the sponge to yield a sponge that leaves less particulate matterand has a longer life than conventional sponges, and a method ofcleaning semiconductor material using the sponges.

2. Description of the Related Art

The use of synthetic sponges, made of polyvinyl acetal for example, iswell known. For example, U.S. Pat. No. 4,566,911 discloses a scrubbingdevice using a polyvinyl acetal elastic material. Conventional spongeshave been shaped in various forms, such as rollers and pads. Thesesponges are used to clean critical materials such as thin film disks,semiconductor wafers, data storage media, and other materials requiringremoval of particles in the 0.1 micron range, i.e. “ultimate cleaning”.Conventional synthetic sponges have a polymer structure with “dead endpockets” formed therein that trap residue and trace amounts of metals.As the sponge wears, these metals can come out of the sponge in the formof particulate matter. Such particulate matter can damage the surfacesthat are to be cleaned. Further, this type of sponge has tiny fibrils inthe pores thereof that are a result of spaces between the pore formingchemical additive during a cross-linking reaction. “Cross-linking” isthe formation of ester bonding between chains of the two adjacenthydroxyl groups that occur with the reaction of polyvinyl alcohol andaldehyde. This reaction hardens and strengthens the resulting material.

U.S. Pat. No. 4,098,728, the disclosure of which is incorporated hereinby reference, discloses a polyvinyl acetal sponge and a method formaking the same. In this method, pore spaces in the sponge are formed bygas bubbles; not pore forming chemical additives such as starch/sulfatecombinations. Because the sponge disclosed in this patent does not haveany starch residue, or the like, it has been particularly useful inmedical applications in which starch residues can cause a foreign bodyreaction when in contact with mucosal tissue.

Also, conventional sponges have been used to form cleaning rollers orthe like having projections formed on a surface thereof. However, theconventional projections have walls that extend perpendicular to anouter surface of the sponge body. Such a configuration creates stressconcentration areas. Accordingly, the projections break off of theroller surface after continued use. Therefore, conventional spongeshaving projections have a short useful life.

Finally, extraction processes, also known as purification processes, areused to remove particulate matter and residue from the sponge before thesponge is used for cleaning. However, conventional extraction processeshave not adequately removed residue to the extent necessary for reliablycleaning semiconductor wafers or similar materials.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome the limitations of theprior art polyvinyl acetal sponges.

It is a further object of the invention to minimize particulate residuein a polyvinyl acetal sponge.

It is still a further object of the invention to extend the life ofpolyvinyl acetal sponges.

The invention achieves these objectives by forming a polyvinyl acetalsponge with a process in which the pores are formed by gas to provide anopen pore structure having no fibrils. Additionally, the sponge ispurified by alternating exposure of the partially cross-linked sponge tosolutions of high pH and low pH. The sponge is sterilized by electronbeam radiation to destroy micro organisms that can cause contaminationof the final product. Residue in the sponge is reduced to 2 ppm or less,preferably 1 ppm or less for many elements, zinc and calcium inparticular.

The resulting sponge material is formed into an industrial sponge havinga cylindrical body and a plurality of projections extending from thebody. The projections are tapered truncated cones having across-sectional radius that decreases with distance from the body. Theprojections of the invention provide increased contact pressure withoutreducing the life of the industrial sponge to unacceptable levels. Thesponge material can be used for cleaning semiconductor wafers or othermaterials by rolling the sponge across a surface of the material in aknown manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a sponge roller device according to apreferred embodiment of the invention;

FIG. 2 is a cross-sectional view along line 2′—2′ of FIG. 1;

FIG. 3 is a partial cross-sectional view taken along line 3′—3′ of FIG.1;

FIG. 4 is a micro photograph of the sponge material of the preferredembodiment;

FIG. 5 is a microphotograph of a typical conventional sponge material;

FIG. 6 is a microphotograph of the conventional sponge illustratingresidual starch particles; and

FIG. 7 is a microphotograph of the conventional sponge at 100×magnification illustrating the bi-refringent colorimetric method used toisolate the starch particles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the invention is illustrated in FIGS. 1-3.Applicant has discovered that the sponge making process disclosed inU.S. Pat. No. 4,098,728, the disclosure of which is incorporated hereinby reference, can yield a sponge material having 100% open pores with no“dead end pockets” and no fibrils. Specifically, the sponge material isa continuous porous web interconnected material lacking fibrils that canresult from spaces between pore forming chemicals during conventionalcross-linking reactions. In conventional sponge materials, fibrils areformed as uncross-linked polymer flows into the spaces existing betweenand around the agglomerated solid particles of starch/sulfate or otherpore forming chemicals. As the cross-linking reaction proceeds, smallincursions into these spaces become solidified as fibrils. FIG. 4 is amicrophotograph of the sponge material used to form sponge roller device10. The sponge material is formed of interconnected thin-walledstructures with holes of substantially circular or oval cross section.The pore geometry of the sponge material does not contain micro fibers,i.e. fibrils, on the walls that define the pores. Also, the pore wallsare attached to each other by a web-like membrane which is itselfporous. In contrast, FIG. 5 illustrates the material used in cleaningand polishing rolls manufactured by Kanebo Corporation and distributedin the U.S. by Rippey, which is a typical conventional polyvinyl acetalsponge material. This material clearly contains tiny fibrils that werecreated by pore forming chemicals during a cross-linking reaction. Thesefibrils can come loose and fall as particulate matter onto the devicebeing cleaned thus reducing the cleaning efficiency of the resutingsponge.

The specific geometry of the pores is a function of the reactionparameters, such as temperature, mixing technique, pore forming method,cure time, reactant concentration, catalyst type and concentration, flowrates, and the like, as described in the above-noted patent. An exampleof the sponge material that can be used for the invention is the same asthat used to form the readily available Merocel Sponge™ and Merocel PVARoller Brushes™ sold by Merocel Corporation which is made by the processdisclosed in the above-noted patent. Additionally, applicant's havefound that known continuous mixing techniques can be used in place ofthe mixing technique disclosed in the patent.

As illustrated in FIG. 1, sponge roller device 10 is molded to form amonolithic cylindrical body having outer surface 12 and a plurality oftruncated conical projections 14 extending from outer surface 12.Projections 14 are disposed at regular intervals in rows 30 and 32 thatare offset from one another, in a direction along the longitudinal axisof the cylindrical body, to result in a staggered arrangement ofprojections 14 as is best illustrated in FIGS. 1 an 2. Preferably,projections 14 extend from outer surface 12 to a height of approximately0.1875 inches as illustrated in FIG. 3. Also, projections 14 preferablyare disposed at intervals to define an angle a of 22.5° 251 betweenradial lines extending through the central axes of adjacent offsetprojections as shown in FIG. 2. Preferably, the diameter of base 18 isat least 20% larger than the diameter of top surface 16. The area ofbase 18 is preferably 40% larger than that of top surface 16.

Each projection 14 has planar top surface 16 that has a diameter of 0.25inches and thus an area of 0.049 square inches. Also, each projection 14has base 18 having a diameter of 0.31 inches and thus an area of 0.073square inches. The taper of projection walls 19, which define the outersurface of the truncated cone and thus are continuously curving, definesan angle 13 that is in the range of 5-20°, inclusive, and is preferablyabout 10° between the linear direction in which the projection extendsfrom the outer surface of the cylindrical body and the central axis ofthe projection as shown in FIG. 3. This continuously tapering wallconfiguration provides a base for the projection that is sufficientlylarge and prevents stress concentration points to prevent reduced lifeof sponge roller device 10 due to broken projections 14. Also, thetapered wall configuration permits foreign mailer on the object to becleaned to enter readily between projections 14 and retained betweenprojections 14.

The sponge material is shaped into the sponge roller device 10 by amolding process. In particular, a polyethylene, ABS, or the likematerial is formed, through molding or the like, into a mold, i.e. afemale version of the outer shape of the sponge roller device, withallowances for shrinkage, the amount of which can be determinedexperimentally. The mold can be made in separate segments and joinedinto one unit. The segments can be interlocked with one another and heldtogether by end caps, or the like. The froth reactant of the spongematerial is injected, or otherwise introduced, into the assembled moldand then the filled mold is placed in an insulated container wheretemperature is controlled during curing. This process is described indetail in U.S. Pat. No. 4,098,728.

The molded sponge material is then subjected to an extraction process toremove any trace amounts of metal and other residues that can causeparticulate matter to fall out of the sponge material. The partiallycross-linked, i.e 60% or less cross-linked, molded sponge roller isalternately exposed to solutions of high and low pH to precipitatemetals from the pore spaces (high pH solutions) and dissolve traceamounts of metals (low pH solutions) from the pore spaces. Cross-linkingin the range of 25-55% yields a strong, yet flexible sponge material.

A first preferred extraction method includes the following steps:

-   -   1(a) soaking the rollers in water to dilute reactants and halt        cross-linking, then washing rinsing and extracting the rollers        until all acid catalyst is neutralized, i.e. a pH between 6.5        and 7.5 is reached; or    -   1(b) soaking the rollers in water and then passing the rollers        through rubber rolls or the like to compress the rollers and        thus extract any solution therefrom;    -   2) soaking the rollers in mineral acid at a concentration of 1        to 20% by weight (preferably between 1 and 5%) and washing and        rinsing until all of the acid catalyst has been removed;    -   3) oxidizing unreacted aldehydes that are within the pore        structure to form carboxylic acids and neutralizing the acids to        soluble salts and washing and rinsing to remove the salts, while        raising the pH beyond 7.0 to precipitates metals;    -   4) washing and rinsing to remove loosely held particulate        matter;    -   5) washing with deionized water, extracting and rinsing, the        last rinse being with distilled water.

The steps above reduce extractable residue and particulate matter tovery low levels (see the test results below) that are suitable forcleaning magnetic disk surfaces or the like. However, for cleaningsilicon wafers, it is desirable that extractable residues of elementssuch as, calcium, zinc, arsenic, barium, cadmium, chloride, chromium,copper, iron, lead, manganese, magnesium, mercury, nitrate, potassium,selenium, silica, silver, and sodium each be reduced to 2 ppm. or less,preferably 1 ppm. or less. Also, it is desirable to reduce extractablesulfate to a value 10 ppm. or less. Reducing the values of extractablecalcium and zinc is particularly important. Applicant has developed anadditional second extraction process, to be conducted in seriatim withthe first extraction process, that reduces extractable residue to thesevalues. The second extraction process includes the following steps:

-   -   1) high turbulence washing and rinsing and then centrifugal        extraction to remove all liquid;    -   2) exposure, through submersion or the like, to an acid solution        of 0.1 to 10% (preferably 0.2 to 2.0%) of a mineral acid such as        sulfuric acid for 1 to 90 hours (preferably 4 to 48 hours).    -   3) more high turbulence washing and rinsing;    -   4) oxidation of reactant aldehydes by soaking the rollers in an        oxidizing bath up to 10% concentration within adjustment for pH        between 7.5 and 11 (preferably between 8.0 and 9.5 in a hydrogen        peroxide bath at 0.05% to 2.0% concentration) for to 96 hours        (preferably 4 to 36 hours).    -   5) repeat of steps 1-4 above until the pantanedione test, or a        similar test, for alhehyde indicates less than 1.0 ppm HCO        equivalent; and    -   6) sterilization by electron beam radiation to destroy        microorganisms, such as mold spores, which can contaminate the        final product.

Steps 1-5 above reduce residue to a very low value. Step 6 eliminatesthe need for pesticide or other additives for sterilizing the roller.These additives require an expensive washout procedure and can furthercontribute to residue.

FIGS. 6 and 7 respectively illustrate residue (potato starch) remainingin the sponge material of the conventional sponge before use. Thewhitish spots in FIG. 6 are residue. FIG. 7 illustrates thebi-refringent colorimetric method used to isolate and detect the starchparticles. Because the invention does not use starch to form pores, thisresidue is eliminated in the invention.

Comparative testing between the sponge material subjected to theextraction processes of the invention and the conventional sponge notsubjected to the inventive extraction processes has been conducted. Thefirst series of tests was a measure of extractible residue contained inthe sponge material. The results of this test are illustrated in Tables1-4 below.

TABLE 1 (Invention Subjected to First Extraction Process Only):COMPONENT INVENTION COMPETITIVE SPONGE Calcium 2.6 ppm 2.6 ppm Chloridenot detected 10 ppm Magnesium .17 ppm .22 ppm Potassium .21 ppm 6.3 ppmNitrate 1.1 ppm 1.3 ppm Nitrite not detected not detected Silicon notdetected not detected Sodium 3.4 ppm 5.8 ppm Total Non 36 ppm 39 ppmVolatile Residues

TABLE 2 (Invention Subjected to First and Second Extraction Process):COMPONENT INVENTION COMPETITIVE SPONGE Calcium 0.55 ppm 2.1 ppm ChlorideNot Detectable 33.0 ppm Fluoride 0.98 ppm 125 ppm Magnesium 0.27 ppm 3.8ppm Potassium 0.16 ppm 16.0 ppm Sodium 1.1 ppm 223 ppm Sulfate 5.4 ppm79 ppm Zinc 0.07 ppm 4.5 ppm Total Non Not Detectable 308 ppm VolatileResidue

TABLE 3 (Invention Subjected to First and Second Extraction Processes):COMPONENT INVENTION COMPETITIVE SPONGE Calcium 0.2 ppm 2.60 ppmPotassium 0.03 ppm 6.30 ppm Nitrate 0.3 ppm 1.30 ppm Total Non NotDetectable 39.0 ppm Volatile Residues

TABLE 4 (Invention Subjected to First and Second Extraction Processes):COMPONENT INVENTION COMPETITIVE SPONGE Arsenic Not Detectable NotDetectable Barium Not Detectable Not Detectable Cadmium Not DetectableNot Detectable Calcium 0.2 ppm 1.6 ppm Chloride Not Detectable 57 ppmChromium Not Detectable Not Detectable Copper Not Detectable 1.1 ppmIron Not Detectable 1.6 ppm Lead Not Detectable Not Detectable ManganeseNot Detectable 0.09 ppm Magnesium Not Detectable 7.4 ppm Mercury NotDetectable Not Detectable Nitrate 0.3 ppm 3.95 ppm Potassium 0.03 ppm 26ppm Selenium Not Detectable Not Detectable Silica Not Detectable NotDetectable Silver Not Detectable Not Detectable Sodium 0.94 ppm 442 ppmSulfate 2.6 ppm 79 ppm Total Non Not Detectable 575 ppm Volatile ResidueZinc 0.04 4.5

From the test results above, it is clear that the invention has far lessextractable residue than the competitive product. Additionallyparticulate analysis testing was conducted on a sponge of the preferredembodiment and a conventional sponge. Both the IES-RP-004 biaxial shaketest and the IES-RP-004 new zero mechanical stress test were used. Thesetest results are presented in Table 5 and Table 6 below.

TABLE 5 (Biaxial Shake Test): Invention Invention Using First UsingFirst Extraction and Second Competitive Process Extraction Sponge OnlyProcesses Number of Particles 338,725 25,301 7026 Released per cm²

TABLE 4 (New Zero Mechanical Stress Test): Invention Invention UsingFirst Using First Extraction and Second Competitive Process ExtractionSponge Only Processes Number of Particles 37,140 1,462 982 Released percm²

The test results above show that the particulate matter released by theinvention is substantially lower than that of the competitive device.Therefore, the invention is less likely to damage or contaminate thesurface that is to be cleaned. The actual amount of residue afterextraction varies somewhat depending on the residue before extractionwhich is a function of the completeness of reactions during formation ofthe sponge.

The extraction processes disclosed above yield a sponge with lowextractable residue. By performing both the first and second extractionprocesses and culling out sponges having residues greater than a desiredvalue for desired elements, a sponge can be made that is particularlysuitable for cleaning semiconductor wafers or other materials that mustbe cleaned very effectively. For example, sponges having 1 ppm or lessof zinc and calcium can be selected for cleaning semiconductor wafers.Prior art extraction does not yield a sponge material having such lowextractable residues for any starting sponge material. Therefore, theinvention provides a sponge that is superior in cleaning semiconductorwafers or the like. Prior art extraction did not attain the low valuesof extractable residue, particularly zinc and calcium, that are madepossible by the invention.

The disclosed extraction processes can be used to purify other spongematerials, such as that of the competitive product. However, the resultsmay not be as good because the prior art has closed pores and fibrils asnoted above. Also, the extraction processes can be applied to spongesrollers that do not have projections or that have different shapedprojections. Further, the extraction process and disclosed spongematerial can be applied to pads or other cleaning apparatus that are notrollers.

The invention has been described through a preferred embodiment.However, it will be apparent the various modifications can be madewithout departing from the scope of the invention as defined by theappended claims.

1. An industrial sponge roller device comprising: a cylindrical bodymade of polyvinyl acetal material; and projections made of polyvinylacetal material formed monolithically with said cylindrical body, saidprojections extending from an outer surface of said cylindrical body,said projections having central axes, respectively, and continuouslycurving side walls, respectively, extending inwardly from said outersurface of said cylindrical body in a linear direction toward and notparallel to said central axes, respectively, to reduce stressconcentration at locations where said projections, respectively,intersect said outer surface of said cylindrical body.
 2. An industrialsponge roller device as recited in claim 1, wherein said projectionseach have a tapered shape of maximum width at said locations where saidprojections intersect said outer surface of said cylindrical body.
 3. Anindustrial sponge roller device comprising: a cylindrical body made ofpolyvinyl acetal material; and projections made of polyvinyl acetalmaterial formed monolithically with said cylindrical body, saidprojections extending from an outer surface of said cylindrical body andhaving central axes, respectively, said projections taperingcontinuously toward said central axes, respectively, in a direction awayfrom said outer surface of said cylindrical body to reduce stressconcentration at locations where said projections, respectively,intersect said outer surface of said cylindrical body.
 4. An industrialsponge roller device as recited in claim 3, wherein said projectionshave side walls, respectively, that extend in a linear direction fromsaid outer surface of said cylindrical body.
 5. An industrial spongeroller device comprising: a cylindrical body made of polyvinyl acetalmaterial; and projections made of polyvinyl acetal material formedmonolithically with said cylindrical body, said projections extendingfrom an outer surface of said cylindrical body, each of said projectionshaving a truncated conical configuration with a central axis, a circularbase intersecting said outer surface of said cylindrical body, acircular top surface at a free end thereof, said top surface having anarea that is less than an area of said base, and sides extending in auniform linear direction from said outer surface to said top surface.